Bread and pastry products are made on the basis of cereals or cereal fractions, which are milled to flour and mixed with yeast or chemical leavening agent, salt and water as key ingredients. Other ingredients such as sugar, fat, milk, oxidants, emulsifiers and enzymes can be added to facilitate processing and/or to improve taste or structure of the baked product and/or its shelf life. The process can be divided in three basic unit operations i.e. i) mixing and formation of a visco-elastic dough by kneading, ii) leavening of the dough through the action of yeast or chemical leavening agents, and iii) baking (Hoseney, 1994).
Arabinoxylan (AX), also referred to as pentosan, is a major constituent of cereal grains, occurring at 5-10% of dry weight of the cereal grains. AX can be divided into either water-extractable AX (WE-AX) and water-unextractable AX (WU-AX), both of which have a similar structure but differ in the level of cross-linking with other natural polymers. In general, AX from cereals consists of a backbone of beta-(1-4)-linked D-xylopyranosyl residues (xylose), some of which are mono- or disubstituted with alpha-L-arabinofuranosyl residues (arabinose). In addition, other substituents, such as ferulic acid, coumaric acid, acetic acid or (methyl)glucuronic acid, are coupled to some of the xylose and/or arabinose residues of AX.
AX is recognized to be important in the production of bread and pastry products. AX from wheat endosperm flour (about 2% of dry flour weight) are estimated to bind 23% of all water in a wheat flour dough (Bushuk, 1966). Addition of increasing levels of WE-AX or WU-AX increases dough development time and dough consistency when water absorption is kept constant. The increase in dough consistency is higher for WU-AX than for WE-AX, while the increase in development time is higher when WE-AX are added (Michniewicz et al., 1991). Both WE-AX and WU-AX increase the Farinograph water absorption (Michniewicz et al., 1991). Farinograph water absorption is also affected by the MW of AX. At similar AX concentrations, HMW (high molecular weight) AX fractions have higher impact on Farinograph water absorption values than LMW (low molecular weight) AX fractions (Michniewicz et al., 1991; Biliaderis et al., 1995; Courtin et al. 1998). Addition of WE-AX to wheat flour enhances bread loaf volume, while addition of WU-AX decreases it (Michniewicz et al., 1992; Courtin et al. 1999).
Endoxylanases hydrolyse internal linkages in the AX chain, thereby affecting the structure and physicochemical properties of AX. Optimal levels of the right type of endoxylanases improve dough and bread characteristics (McCleary, 1986; Rouau et al., 1994) and some endoxylanases are therefore currently marketed as improvement agents for baked products. Addition of endoxylanases at optimal doses during bread-making has the following desirable effects (Rouau et al., 1994):                increased viscosity and elasticity of the dough;        increased loaf volume after baking;        softer crumb structure (reduced crumb firmness).        
The beneficial effect of endoxylanase addition in bread-making is due to transformation of WU-AX into enzyme-solubilized AX (ES-AX, i.e. a fraction of AX that has become water-extractable through the action of exogenously added endoxylanases on the originally present WU-AX). Hypothetical explanations for the causal effect of increased levels of ES-AX or reduced levels of WU-AX on the above described improvement of dough and bread characteristics include the following (Rouau et al., 1994; Courtin and Delcour, 2002):                reduction of the water-binding capacity of WU-AX, resulting in a redistribution of previously bound water over other dough components such as gluten, thus improving the development of the gluten network and hence dough extensibility;        reduction of WU-AX levels that destabilize the dough structure because they can form physical barriers for the gluten network and can perforate gas cells, resulting in coalescence and decreased gas retention;        increased viscosity of the dough aqueous phase due to higher levels of ES-AX, thus stabilizing water films in gas cells during dough fermentation;        slowing down of the rate of diffusion of carbon dioxide out of the dough system due to improved mechanical strength or reduced gas permeability of the liquid films in the dough, resulting in higher gas retention during dough fermentation, and thus increasing loaf volume after baking.        